Post-deflection electrostatic electron lens systems incorporated in conventional cathode-ray tubes typically perform two distinct functions. First, the lens system magnifies the amount of the electron beam deflection produced by the deflection structure of the CRT to provide an image of desired size on the display screen. Second, the lens system accelerates the electrons in the electron beam by developing a high intensity electric field between the exit end of the lens system and the display screen. This increases the energy of the electrons and thereby produces a brighter image on the phosphorescent screen.
Certain cathode-ray tubes are provided with microchannel plates adjacent their display screens to obtain greatly enhanced visual and photographic writing speeds. Such a CRT is used, for example, in the Model 7104, 1 GHz oscilloscope manufactured by Tektronix, Inc. A microchannel plate, or MCP, is a two-dimensional array of individual channel electron multipliers, which generate from 1,000 to 10,000 or more electrons for each input electron received. Located with its output face near the inner surface of the phosphorescent display screen of the CRT, the MCP multiplies beam electrons striking its input face to produce a trace of greatly increased brightness on the display screen. Among other advantages, this enables the viewing of extremely fast traces that otherwise would not be visible on the display screen of the CRT.
Mesh lenses are commonly used in post-deflection acceleration (PDA) cathode-ray tubes to increase deflection sensitivity and to prevent the penetration of high voltage accelerating fields into the low voltage deflection regions of such tubes. A conventional accelerating mesh lens would be unsuitable, however, for use in a cathode-ray tube having a microchannel plate. The reason is that the lens mesh intercepts some of the electrons exiting the deflection structure and creates additional electrons by way of secondary emission. The secondary emission electrons are accelerated toward the phosphorescent screen and produce spurious light patterns, typically in the form of a halo, and degrade the display contrast. The use of a microchannel plate in association with an accelerating mesh lens would, therefore, function to multiply the number of secondary emission electrons and thereby further degrade the display contrast.
To prevent the creation and thereby the multiplication of secondary emission electrons, it would be necessary to employ a "meshless" scan expansion lens, such as the rectangular box-shaped lens that is the subject of U.S. Pat. No. 4,124,128 of Odenthal, or the interdigitated tubular quadrupole lens shown and described in U.S. Pat. No. 4,188,563 of Janko. The scan expansion lenses of Odenthal and Janko do not employ mesh elements and, as a consequence, do not create secondary emission electrons. Both of these lenses suffer, however, from the disadvantages of being difficult to manufacture and align.